The most common method of separating solids contents from a pulp is the filtering of the pulp by means of more or less fine pored filter media. A main disadvantage of these methods is that the pores of the filter media relatively quickly peg by solid matter particles from the pulp so that a severe hydrostatic pressure must be exerted onto the pulp in order to pass liquid through the filter medium. This hydrostatic pressure on the one hand leads to the fact that the particles are pressed even more into the pores of the filter medium thus causing the filter efficiency to deteriorate even further.
These disadvantageous effects can be reduced in that the filtering process is performed in many stages using progressively reduced pore sizes of the filter media. This, however, requires great expenditure.
A different help is the so-called cross-flow filtering in which the pulp is kept in a movement extending transversely to the surface of the filter medium to prevent the deposition of solid matter particles on the surface of the filter medium. Filter devices operating in this manner are for instance known from EP 0 178 389 A1. Also in these filter devices the formation of a filter cake on the filter media is basically unavoidable, thus influencing the filtering efficiency.
A further filter device operating by cross-flow is known from EP 0 226 659 A. In this filter device, a rotor rotating in the pulp chamber and which keeps the pulp in motion during the filtering process, is displaceable upon termination of the filtering process against the unmoved filter medium so that by the aid of this rotor the filter cake unavoidably growing on the filter medium can be mechanically pressed out before being removed from the filter device. The operation of this device therefore consciously provides the separation of the solid matter particles on the filter membranes and requires cyclic interruptions which are necessary to press out the filter cakes that were produced and to remove them from the filter device. The generation of filter cakes during the filtering process moreover requires significant hydrostatic pressure in order to achieve satisfactory filter efficiencies.
Thus, the object of the invention is to provide a method and an apparatus by means of which a separation of solids contents from a pulp is possible during continuous or more or less continuous operation and wherein an increased efficiency can be obtained without a high hydrostatic pressure having to be exerted onto the pulp.
This object is solved with respect to the method by the features cited in claim 1. Advantageous embodiments of the invention are subject matter of the dependent claims.
According to the invention the pulp is only concentrated in the filter device. This enables the pulp to be kept in motion during the entire treatment process in the filter device used for this purpose so that due to the pulp flow running transversely to the surfaces of the filter media a deposition of solid matter particles on the surfaces of the filter media is fully or almost fully prevented. Thus, the differential pressure at the filter media is kept small. Preferably the speed of the cross flow of the pulp is set especially high. In a preferred embodiment of the invention a pump flow is generated in the pulp which does not only lead to a high cross flow speed but also generates a high dynamic pressure in the pulp at the filter membranes so that the differential pressure of the membranes is overcome without requiring the pulp to be exposed to a hydrostatic pressure.
When the pulp chamber is set under hydrostatic pressure, which is also possible, this pressure can be set low compared to the prior art. Then, it only serves the purpose of ensuring a sufficient filtrate flow through the filter media, since it does not have to overcome a filter obstruction.
If during the filter operation a predetermined solid matter concentration of the pulp in the filter device is achieved, this concentrated pulp is mechanically pressed out. For this purpose it is first of all taken off the filtering process.
The concentrated pulp may be conditioned by an additive to progress the pressing process, e.g. by the aid of milk of lime in order to generate coagulation.
The subsequent mechanical pressing process is not a filtering process but shall only serve for the rough drainage of the concentrated pulp. Accordingly the discharge from the pressing process is not free from particles and is therefore returned into the filtering process.
The pressing cake generated by the mechanic pressing process is then mechanically comminuted and the agglomerates produced thereby can, if desired, be kept in a whirling condition so that the remaining moisture is withdrawn from them. This drying process can be accelerated by the influence of vacuum pressure or by heat. An ultraviolet irradiation can also be acted upon the whirling particles in order to kill possibly existing germs.
To carrying out the method, a plurality of pressing devices of smaller capacity can be assigned to a filter device of a greater capacity so that the filter device can be more or less continuously supplied by fresh pulp and can be freed from the particle concentrate, wherein the pressing devices, preferably pressing dryers are used in a chronologically staggered manner.
The solid matter particle concentration degree reached in the filter device can for instance be determined by optical means detecting the light permeability of the pulp, or by measuring the driving power which is required to keep the pulp in the filter device in motion.
The operation is especially economical when during the drying process the heating of the particles in the whirl chamber is performed by process heat, for example the heat that is produced during the filtering process due to inner friction in the pulp and which is transferred to the filtrate. Then the filtrate can be used for heating the particles in a whirl chamber in a manner that the walls of the whirl chamber are heated by the filtrate. This filtrate can be conditioned before or after utilizing its heat, for instance its pH value may be set in order to be gentle for the apparatus and the environment.